In general, transmitted data is received by a receiver along different paths (i.e., “multipath”), primarily due to signal reflections off buildings, hills, mountains, trees, etc., located between the transmitter and the receiver. This effect is magnified when one of the system terminals is a mobile terminal (such as in an automobile). FIG. 1 illustrates an example of multipath between a base station 100 and a mobile terminal 102, in which signals are transmitted, for example, directly (104), reflected off a mountain (106), and reflected off a building (108).
Because the different multipath components travel different distances before reaching the base station 100, they reach the base station 100 at different times.
Voice transmission, for example, is transmitted continuously across a narrow bandwidth wireless channel. On the other hand, packet data transmission occurs only sporadically (by definition, based on the intermittent transmission of data “packets”), but uses a large bandwidth when it does occur. A large number of packet data channels can share a wireless channel, because only one packet data channel having data to transmit will be actively sending data packets at any one time, whereas all other packet data channels will be quiet. Thus, the packet data channels in effect take turns sending packet data across the wireless channel.
In view of the foregoing, a particular problem occurs in packet data transmission between a mobile terminal and a base station. Specifically, the mobile terminal transmits packet data during an active period and is silent during an inactive period. The problem arises because the mobile terminal, by definition, moves relative to the base station, during both the active and inactive periods.
Finding new multipath components during the movement of the mobile terminal during an active period (i.e., while packet data is being transmitted) is processed in a conventional manner in what is known in the art as a “standard search.” For example, it is conventionally known to use a “rake receiver” in base-band receiver systems, using code-division multiple access (CDMA). Each rake receiver has rake fingers, each of which independently tracks a respective received multipath component. Each rake finger adaptively cancels delay spread between multipath components, adjusts the phase between the multipath components, and equalizes the level of output of the received signal. The rake receiver can therefore receive simultaneous signals corresponding to each multipath component from the same CDMA carrier.
A CDMA base-band receiver also includes a searcher subsystem for finding new multipath components by scanning received data. The multipath components may be particularly indicated by pre-defined symbol patterns called pilot symbols, which are known to the receiver. Alternatively, the received energy at different delay offsets may be measured in the search window to locate the multipath components. The CDMA base-band receiver also includes a rake finger management system for setting up and tearing down rake fingers based on information provided by the searcher subsystem.
FIG. 2 is a flow chart illustrating a conventional approach to moving between active and inactive periods of data transmission at the receiver. Detection of multipath components during movement of the mobile terminal during an active period (i.e., while packet data is being transmitted) is processed in a conventional manner by the searcher and rake finger management subsystems of the base station. More specifically, once a valid multipath component is acquired and assigned to a rake finger in the rake receiver, standard searches are performed in a known manner, using a standard search window associated with the rake finger having the strongest power. Standard search requests (indicated by 200 in FIG. 2) are performed until the mobile terminal goes into the inactive state, at which time the receiver loses contact with the mobile terminal. Standard search requests are performed in accordance with known methods in a variety of ways.
If data transmission stops (i.e., if an inactive period starts), as determined at 202 in FIG. 2, an acquisition search request is processed, as indicated by 204. During the inactive period the mobile terminal cannot be tracked because there is no data being transmitted, including pilot symbols. Therefore, when a mobile terminal switches from inactive to active, an acquisition search 204 is needed to determine the location of the mobile terminal. Acquisition searches are performed continuously during the inactive period because receiver does not know when the transmitter will have data to transmit. The receiver must therefore be on the “lookout” for an incoming signal.
An example of an acquisition search process 204 used by a conventional searcher subsystem is schematically illustrated in FIG. 3. As shown, a sample of a detected input signal 302 is multiplied in a known manner with a sample of a reference signal 304 equal in length to the input signal sample, using a multiplier 306. Both the input signal 302 and the reference signal 304 are complex digital (e.g., (1±j) and −(1±j)). Accordingly, the result of multiplying input signal 302 and reference signal 304 results in a plurality of values which are summed by adder 308 in a known manner, thereby resulting in a certain total value over the entire sample. That total value is stored, for example, in memory register 310.
Thereafter, the reference signal 304 is shifted by an arbitrary amount that is less than one chip (for example, one ½-chip) relative to the input signal 302 in a known manner by signal shift controller 312. The process of multiplying the input signal 302 and reference signal 304 (now shifted relative to each other) is then repeated. The summed value of that operation is also stored in memory register 310. The input signal 302 and reference signal 304 are then shifted incrementally again relative to each other.
The process of shifting the input and reference signals, multiplying the signals, and storing the result is repeated over the entire “width” of the conventional acquisition search window (which is, for example, a certain number of ½-chips). The conventional acquisition search window corresponds to a radius of the cell associated with the receiver (e.g., 10 km).
Once the search over the entire acquisition search window is complete, the highest value stored in memory register 310 is identified in a known manner by a maximum value detector 314. That highest value is compared to a predetermined threshold value in a known manner by a discriminator 316. Exceeding the threshold value corresponds with acquisition of a new viable multipath component, thus indicating the end of the inactive period and the start of a new active period (see step 206 in FIG. 2). Therefore, a new standard search request (200 in FIG. 2) is started, as discussed above.
Not exceeding the threshold value corresponds with a determination that data transmission has not restarted. The process then returns to perform another conventional acquisition search (204 in FIG. 2).
Generally, the conventional acquisition search window is substantially wider than the standard search window (to account for uncertainty in the location of the mobile terminal, which can be anywhere in the cell). However, a wide acquisition search window leads to a longer search time (because more cycles of signal shifting, multiplication, and value storage are required), which leads to a longer acquisition time for acquiring the channel in question. The width of the conventional acquisition search window is not adaptively associated with, for example, the length of the mobile terminal inactive period or the last known location of the mobile terminal.
Long search times are problematic because when the mobile terminal enters an active state from an inactive state, a preamble is sent across the channel to the base station, prior to packet data transmission. Once transmission of the preamble is complete, the channel must be acquired and the channel and the receiver must be ready to receive the packet data transmission. If the receiver fails to acquire the channel prior to packet data transmission, part of the packet data is lost. It is therefore important to have a relatively fast acquisition search algorithm that is completed before the preamble is over. In the conventional acquisition search, time is consumed by completing a full search over the entire conventional acquisition search window, which delays the receiver being able to ready itself to receive the packet data from the mobile terminal.